1. Field of the Invention
This invention generally relates to a radio communication apparatus having a unit for measuring the distance between two stations. This invention particularly relates to a mobile radio communication apparatus having a unit for measuring the distance between a mobile station and a base station. This invention also relates to a mobile radio communication apparatus having a unit for detecting the timing of the reception of a reference signal transmitted from a base station.
2. Description of the Related Art
Japanese published unexamined patent application 7-38951 discloses a method of detecting the position of a portable station (a mobile station) relative to the positions of base stations. According to the method in Japanese application 7-38951, a position detection inquiry signal is transmitted from one of the base stations to a portable station at a predetermined time period. The portable station sends a position detection answer signal when receiving the position detection inquiry signal. The position detection answer signal contains identification information related to the portable station. The position detection answer signal is received by at least three base stations close to the portable station. In each of the three base stations, the electric field strength of the received position detection answer signal is measured, and the distance between the portable station and the base station is calculated on the basis of the measured electric field strength. The position of the portable station relative to the positions of the base stations is estimated from the calculated distances between the portable station and the three base stations.
In Japanese application 7-38951, the distance between the portable station and the base station is calculated on the basis of the electric field strength of the portable-station signal received at the base station. Since the electric field strength of the portable-station signal received at the base station depends on not only the distance between the portable station and the base station but also other factors. Generally, some of the other factors significantly affect the electric field strength of the received signal. Accordingly, in Japanese application 7-38951, the calculated distance between the portable station and the base station tends to be inaccurate in certain conditions.
Japanese published unexamined patent application 8-179028 relates to detection of the present position of a mobile station in a cellular telephone network. In Japanese application 8-179028, a mobile station has a function of measuring the levels of received radio signals transmitted from at least three adjacent base stations including a traffic base station. When the mobile station requests information of its present position, the traffic base station instructs the mobile station to report the measured signal levels. The traffic base station calculates the losses of propagation of radio wave between the mobile station and the base stations on the basis of the measured signal levels reported by the mobile station. Then, the traffic base station calculates the distances between the mobile station and the base stations on the basis of the calculated propagation losses. The present position of the mobile station is estimated from the calculated distances between the mobile station and the base stations.
In Japanese application 8-179028, the distance between the mobile station and the base station is calculated on the basis of the level of the base-station signal received at the mobile station. The level of the base-station signal received at the mobile station depends on not only the distance between the mobile station and the base station but also other factors. Generally, some of the other factors significantly affect the level of the received signal. Accordingly, in Japanese application 8-179028, the calculated distance between the mobile station and the base station tends to be inaccurate in certain conditions.